Depends on the filter. Once a month is probably too long of a wait unless you have very small bioload. But with plant detrius I find I often have to clean at most every few weeks on my biggest filter and weekly on my smaller filter.
I have (2) Eheims: a 2028 and a 2017. I have been cleaning about once every (2) months. Seems like I need to do it more often. I know the 2028 has a flow indicator but I have a shelf in the stand that blocks the viewing of it.
I only clean my hob about once a month. By that time there's lots of brown stuff in it from dead plant leaves so maybe I should do it more often. If my plants are not shedding a lot of leaves, though, once a month is no problem for my setup.
Monthly on all my filters, ie. sumps and canisters. Bio load is medium to high. Filters are not to bad at the month mark and could probably be stretch out longer if needed. Monthly just works in to my schedule.
I agree with Mooner. I have an Eheim 2028 on my 100g, I tend to check it out approx once a month. I generally quickly rinse the coarse sponge & 3 trays of biomedia to get rid of the course rubbish. I replace the final white foam as well with polyester filling from the local fabric store (much much cheaper than the original Eheim sponges). Then it goes straight back on to the tank. The murky contents are usually tipped into a large container where my wife will spend the next hour saving the fry, baby ghost shrimp (last time she found 68) & others that have found their way into the filter.
That's the easy bit...cleaning all the hoses (including UV, heater housing & the pickups & piping running under the substrate) is another story...